1. Field of the invention.
The invention relates to a pressurized fluid storage tank adapted to be installed at a slight depth underground.
The storage tank according to the invention is more particularly applicable to the storage of fuel gas in large quantities.
The use of liquefied gas tanker ships has high-lighted the need to have high capacty tanks available for the storage of pressurized fuel fluids. Tanks of this kind can at one and the same time serve as storage reserves near gas terminals, and as buffers enabling irregularities in the demands on the system to be attenuated.
The form in which the fluid is stored governs to a large extent the technology of the storage tanks.
2. Description of the prior art.
From the technological point of view of distinction is made between low-pressure storage tanks and high-pressure storage tanks.
Gasholders belong to the low-pressure technique. They consist of a dome supported by concentric rings adapted to slide vertically in relation to one another, the whole arrangement, whose volume is variable, being supported by the internal pressure of the gas. Tanks of this kind, which are often additionally supported by an external metallic structure, are inesthetic and, because of their dimensions, constitute easily located targets for ill-intentioned persons.
In cryogenic storage tanks the gas is held in liquid form at very low temperature, thus enabling it to be stored under pressures close to atmospheric pressure.
These cryogenic tanks are protected by a double casing: an outer casing protecting the tank against external agents and a second casing able to hold the liquefied gas and reesistant to very low temperatures (of the order of -40.degree. C. to -160.degree. C.) prevailing in the tank. Between these two casings an insulating material is disposed in order to restrict thermal losses through the tank. Losses of cold through the casing are generally compensated by controlled evaporation of the fluid contained in the tank. The cost of producing these tanks and also the cost of operating them are high.
For high-pressure storage tanks a spherical shape is generally adopted, since the sphere is the technically optimum shape.
However, because of their dimensions the spherical casing and its external reinforcement constitute a prominent feature in a landscape. Like gasholders, these spherical tanks therefore constitute a visual embarrassment and a preferential target for malevolent action. In addition, these tanks are exposed to the direct action of atmospherical agents, such as solar radiation, which may give rise to excessive thermal and mechanical streses in them.
Another way of withstanding the considerable pressure produced by the stored fluid is to place the tanks in a medium which itself exerts a counterpressure on the walls.
This solution is for example applied by excavating deep cavities in clay soil. This material is gastight and the weight of the soil and the pressure of the phreatic water develop the counterpressure which ensures the stability of the tank.
In other types of soil the counterpressure can be exerted by the phreatic water alone; in this case the wall of the underground tank is lined with an impervious covering.
Deep underground storage tanks have the advantage over the other types of tanks mentioned of being inconspicuous and of integrating harmoniously into the environment.
Their disadvantage is above all the high cost of excavation. In addition, the subsoil is not always suitable for the installation of storage tanks of this type.
It has therefore been attempted to produce a type of storage tank which is just as inconspicuous, out of sight and out of reach where malevolent action is concerned as a tank installed at a great depth underground, while nevertheless its operating cost are identical to those of a spherical surface tank.
The aim of the invention is therefore to be able to store pressurized fuel fluids under excellent conditions of reliability.
Another aim of the invention is to effect this storage under good economic conditions.
Yet another aim of the invention is a storage tank of inconspicuous shape and adapted to be covered by a mound or buried at a shallow depth.